A Rotor blades

The rotor blades of the Lagerwey LW-50/750 - APX48/750 - are designed by the Stevin Laboratory of Delft University of Technology, The Netherlands [242], and manufactured by Aerpac Special Products B.V., Hengelo, The Netherlands [241]. The rotor blades are designed for both (full-span) pitch-controlled and (active) stall regulated 3-bladed wind turbines.

The APX-45/APX-48 rotor blade consists of two main parts: a 3.75 m long non-aerodynamic part where the cylindrical contour is transformed into an aerodynamic shaped root aerofoil, and an 18 m long aerodynamic part. The length of both blades is 21.75 m (the APX-45 blade is extended at the root to achieve an APX-48 blade). The primary aerofoils were selected to suit a stall controlled wind turbine (i.e. non-optimal for variable speed wind turbines like the Lagerwey LW-50/750). The optimum tip speed ratio is set to 7 with a maximum blade tip speed of 60 meters per second.

The blades are mainly made of glass fibre reinforced epoxy (GRE). A 12° twist is built in to keep the angle of attack constant over the length of the blade. The main technical data of these rotor blades is summarized in Table A.1 [155].

Fig. A.2 shows the chord length, the chord thickness, and chord ratio (which is defined as the chord thickness divided by the chord length) of the APX-45/APX-48 rotor blade as function of the radial position along the blade. Detailed aerodynamic design of the APX-45 and APX-48 rotor can be found in Van Rooij and Timmer [242].

For the purpose of illustration, Fig. A.3 shows a 3-D impression of the APX-45/APX-48 rotor blade.

Figure A.3: A 3-D impression of the APX-45/APX-48 rotor blade. By courtesy of R. van Rooij, Delft University of Technology, The Netherlands.

The contours of the primary aerofoils (i.e. shape of a cross-section of a rotor blade) of the APX-45/APX-48 rotor blade are shown in Fig. A.4. Observe that the relative thickness decreases from root to tip. Thicker aerofoils near the blade root provide greater strength, and can do so without seriously degrading the overall performance of the blade.

Figure A.4: The contours of the primary aerofoils of the APX-45/APX-48 rotor blade. The relative thicknesses are from top to bottom 18.0%, 21.1%, 25.0%, and 30.3% respectively.

The eigenfrequencies obtained from the full-scale modal test performed by the Stevin Laboratory of Delft University of Technology [156, 241] are listed in Table A.2.

Mode

[rad/s]

1si flap

11.69

1si lead-lag

17.34

2 nd flap

33.62

2 nd lead-lag

51.33

1si torsional

122.5

Table A.2: Rotor blade non-rotating eigenfrequencies from a full-scale modal test performed by the Stevin Laboratory of Delft University of Technology.

The figures indicate that the blades are torsionally rigid. Friedman [67] states that this is typical for wind turbines.